Overriding A System.xml file for adding addition settings to an exist module in a new module.

Module name is (Namespace_Modulename)

Place this code to config.xml

<?xml version="1.0" encoding="UTF-8"?>
        <config>
            <modules>
                <Namespace_Modulename>
                    <version>0.1.0</version>
                </Namespace_Modulename>
            </modules>

            <adminhtml>
                <acl>
                    <resources>
                        <all><title>Allow Everything</title></all>
                        <admin>
                            <children>
                                <system>
                                    <children>
                                        <config>
                                            <children>
                                                <modulename>
                                                    <title>title</title>
                                                </modulename>
                                            </children>
                                        </config>
                                    </children>
                                </system>
                            </children>
                        </admin>
                    </resources>
                </acl>
            </adminhtml>

            <global>
                <helpers>
                    <modulename><class>Namespace_Modulename_Helper</class></modulename>
                </helpers>
            </global>
        </config>

Place this code to system.xml

<?xml version="1.0" encoding="UTF-8"?>
        <config>
          <sections>
            <namespace> --> Place your Section Here
              <groups>
                <homepagetabs translate="label" module="modulename"> --> Place your Group Here
                  <fields>
                    <enable_disable_tab4 translate="label"> --> Place your Field Here
                      <label>Product tab 4</label>
                      <frontend_type>select</frontend_type>
                      <sort_order>13</sort_order>
                      <source_model>adminhtml/system_config_source_yesno</source_model>
                      <show_in_default>1</show_in_default>
                      <show_in_website>1</show_in_website>
                      <show_in_store>1</show_in_store> 
                    </enable_disable_tab4>
                  </fields>
                </homepagetabs>
              </groups>
            </namespace>
          </sections>
        </config>

Note : Change as per requirement like change groupname,sectionname,fieldname.